1. Field of the Invention
The present invention relates generally to antenna switch modules which are used to have a same antenna shared by different paths of a radio-frequency electronic transceiver device.
The present invention more specifically applies to cellular phone type mobile telephony devices that are capable of operating over different frequency bands.
2. Discussion of the Related Art
FIG. 1 is a schematic block diagram of a conventional example of a multistandard mobile phone 1 capable of operating in different frequency bands (GSM-900 MHz, DCS-1800 MHz, PCS-1900 MHz).
For simplification, not all the elements of mobile phone 1 have been shown in FIG. 1. Such a mobile phone is generally powered by a battery (not shown) and comprises one or several radio-frequency signal processing electronic circuits 11 (RF) (generally called radio-frequency transceiver heads) comprising as many transmit-receive paths as there are frequency bands capable of being processed by the phone. The radio-frequency transceiver heads (circuit 11) comprise filtering elements, not shown, adapted to each involved frequency band. In FIG. 1, it is assumed that circuit 11 comprises two transmit paths Tx GSM and Tx DCS/PCS intended for GSM and DCS or PCS transmissions, and three receive paths Rx GSM, Rx DCS, and Rx PCS intended for GSM, DCS, and PCS receptions. In practice, the DCS and PCS transmit paths may be confounded, due to their closeness in frequency.
An antenna switch module 2 (ASM) comprises as many input/output terminals 21, 22, 23, 24, and 25 as the transceiver head circuit comprises paths. Switch 2 selectively connects one of terminals 21 to 25 to a common terminal 27 connected to an antenna 15 of telephone 1.
A first known type of antenna switch module uses a so-called “quarter wave” technology in which high-frequency diodes are combined with λ/4 series impedances (one quarter of the wavelength of the central frequency of the frequency band of the involved path) to perform the switching.
High-frequency diodes are characterized by the fact that their semiconductor junction (PN) is made to avoid rectifying the signal at relatively high frequencies (typically, several hundreds of MHz). Such diodes exhibit high minority carrier lifetimes. Thus, when they are forward biased, even under a low continuous biasing, the diodes conduct the RF signals in both directions, without rectifying them, since they do not have time to block on high-frequency biasing inversions. Conversely, when they are reverse-biased or even at 0 V, due to their low capacitance, the diodes block the RF signals. In practice, such diodes are formed with an intrinsic area (PIN diodes), for example, in an epitaxial layer.
The diodes are used to control the conduction of the paths in the transmit direction (Tx) while the quarter-wave circuits are used as filtering elements for blocking the transmit signals Tx which are not intended for paths Rx.
A disadvantage of circuits combining high-frequency diodes and quarter-wave impedances is that they require many passive elements and many diodes, which increases the bulk.
Another disadvantage is that the presence of the quarter-wave impedances in series increases insertion losses of the antenna switch module.
However, such a structure has the advantage of not generating too many harmonics capable of corrupting the useful signals.
FIG. 2 shows a second conventional example of a so-called “common cathode” antenna switch module 2.
This switch comprises, between terminal 27 intended to be connected to antenna 15 and each terminal 21 to 25 intended to be connected to a radio-frequency transceiver head of circuit 11, a high-frequency diode D1, D2, D3, D4, or D5 having its cathode connected to terminal 27. For simplification, the transmit and receive paths will be designated hereafter as Tx1, Tx2, etc. and Rx1, Rx2, Rx3, etc.
In practice, between each terminal 21-25 and the anode of the concerned diode, a connection capacitor C1, C2, C3, C4, C5 is present. Further, each anode is connected to a bias circuit formed of an inductive and/or resistive element L1 to L5 in series with a switch K1 to K5 to individually control the biasing of the different diodes by applying thereto either a positive voltage V+, or the ground or a negative voltage. On the side of antenna 15, a connection capacitor C15 connects terminal 27 to the antenna, and an inductive element L15 in series with a resistor R15 grounds terminal 27. The selection of the path (for example, Tx1) is performed by applying a positive voltage V+ on the anode of its diode (for example, D1) to bias it forward while the other diodes are all reverse-biased and block the other paths. Voltage V+ must be sufficient to turn on a junction (at least on the order of 0.7 volt) and is in practice a positive voltage available in the device (for example, 2.7 volts). Resistor R15 is used to dissipate the D.C. component (voltage V+ decreased by approximately 0.7 volt) of terminal 27. This dissipation may be transferred (partially or totally) onto resistors in series with inductances L1 to L5.
An advantage of a circuit with common cathodes is that it is less bulky, due to the small number of passive components with respect to the preceding solution.
However, a disadvantage is that each blocked diode (and thus four diodes out of five in the example of FIG. 2) generates harmonics of order 2 having non-negligible amplitudes when the selected path is a transmit path.
A first solution for reducing these harmonics is to increase the resistivity and the thickness of the epitaxial layer of the diodes. This however increases switch insertion losses.
A second solution would be to apply negative biasings to the diodes which are desired to be reverse-biased to improve their blocking. This however requires negative and positive bias voltages, while the two biasings are not necessarily available in the concerned devices. Such is especially the case for mobile phones.